Cryptographic Hash Function . If an attacker could easily reverse a hash function, it would be totally useless. Also, many hash functions (including SHA-1 and SHA-2) are built by using a special-purpose block cipher in a Davies–Meyer or other construction. The use of cryptographic salt prevents some attacks, such as building files of precomputing hash values, e.g. Lists of common passwords are widely circulated and many passwords are short enough enough that all possible combinations can be tested if fast hashes are used. A hash function also has an initial state provided for when zero blocks have been processed. There are several methods to use a block cipher to build a cryptographic hash function, specifically a one-way compression function. Similarly, the key expansion of the HC-128 and HC-256 stream ciphers makes heavy use of the SHA-256 hash function. SHA-3 provides the same output sizes as SHA-2: 224, 256, 384, and 512 bits. [5] The weaker assumption is always preferred in theoretical cryptography, but in practice, a hash-function which is only second pre-image resistant is considered insecure and is therefore not recommended for real applications. Whirlpool is a cryptographic hash function designed by Vincent Rijmen and Paulo S. L. M. Barreto, who first described it in 2000. The output of a password hash function can also be used as a cryptographic key. For example, a CRC was used for message integrity in the WEP encryption standard, but an attack was readily discovered which exploited the linearity of the checksum. BLAKE3, an improved version of BLAKE2, was announced on January 9, 2020. Password verification commonly relies on cryptographic hashes. ��j�u)�T!ԟ�d�[W��k��r=���~�j#�(n���q V�����xM�Hto�N��r!��@�em����Ȃ�@��� �ne��V�� xKI2D��İT48��D�^)ǹ�H� O�ŸT=� X���K�K9�'#㺍{K�Lg+p�5I��r8����i �� ��p�2/��O�X�ּ�|��Zw��d?o�w�]#U�m�D Password hash functions that perform key stretching – such as PBKDF2, scrypt or Argon2 – commonly use repeated invocations of a cryptographic hash to increase the time (and in some cases computer memory) required to perform brute force attacks on stored password hash digests. Often this is done by first building a cryptographically secure pseudorandom number generator and then using its stream of random bytes as keystream. Pseudorandom number generators (PRNGs) can be built using hash functions. In practice, collision resistance is insufficient for many practical uses. One popular system – used in Bitcoin mining and Hashcash – uses partial hash inversions to prove that work was done, to unlock a mining reward in Bitcoin, and as a good-will token to send an e-mail in Hashcash. Therefore, cryptography requires one-way hash functions. SHA-384 and the lesser-known SHA-512/224 and SHA-512/256 are all variants of SHA-512. Because cryptographic hash functions are typically designed to be computed quickly, special key derivation functions that require greater computing resources have been developed that make such brute force attacks more difficult. Hash functions are one way implies that given that hash of a particular input, it is really difficult (practically impossible) to reconstruct the input (completely or partially). In other words, if a hash function h produced a hash value z, then it should be a difficult process to find any input value x that hashes to z. It was created by Jack O'Connor, Jean-Philippe Aumasson, Samuel Neves, and Zooko Wilcox-O'Hearn. Common graphics processing units can try billions of possible passwords each second. A function meeting these criteria may still have undesirable properties. MD5 was designed by Ronald Rivest in 1991 to replace an earlier hash function MD4 and was specified in 1992 as RFC 1321.

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